All wastewater treatment facilities generally utilize grit handling and removal equipment to isolate and/or remove the harmful coarse solids contained in the waste stream flow and eliminate them from downstream processes. Utilizing this equipment also aids in reducing maintenance costs and grit related operational difficulties. This includes protecting sludge pumps, piping, centrifuges, etc., from the scouring action of grit, and preventing grit from reducing a plant's overall efficiency due to clogged sumps and pipes in addition to build-ups in fluid channels, settling basins, flocculation tanks and digestion tanks.
Grit contained in grit slurries, whether removed by mechanical means such as bucket elevators, airlifts, water eductors or pumps must be dewatered and washed to produce a product which is inoffensive and can be easily handled and removed from the facility. Two pieces of equipment were commonly used in plants for this purpose, grit washers and grit separators or classifiers.
SW grit washer units dewater and wash grit particles to a low putrescible content, primarily free of bacteria, fungi, foul smelling odors. These units were traditionally offered in various sizes related to maximum flow rate the units could handle and still provide effective solids separation and removal. The sizes were developed to handle the commonly captured grit size of 65 mesh, approximately 235 microns, or greater and utilized tank surface area and settling rate (related to tank detention time) methodology for sizing specific projects.
A second design called a SW-C grit washer allowed much higher flow rates to the washer unit by using a first stage cyclone grit separator to deliver the grit slurry to the washer. The SW tank sizes (5 GPM TO 40 GPM) could then remain smaller but the overall system could handle the higher flow rates. The overflow from the cyclone separation process is typically returned to the upstream process flow. Since the underflow from the cyclone is at a significantly lower flow rate of approximately 5–10% and has significantly higher grit content per gallon of flow, the tanks typically require additional make-up water to be added to enable proper washing.
Grit classifier units were designed to handle low to moderate solids loads in a slurry, which is being de-slimed, dewatered or washed. The surface area required to effect the desired separation determines the size of the tank. As slurry is discharged into the tank, the flow contained solids settle out of suspension and the clarified overflow passes over an adjustable weir into an effluent box. Again, tank sizes were originally based on handling grit sizes of 65 mesh or greater.
In both grit washers and grit classifiers, a screw is utilized to convey solids to a discharge point, and to agitate the fluidized bed of materials, releasing entrapped slimes for removal. Sizing of the screws and speeds utilized are dependent upon expected volumes of grit and capacity output as required by the application. Variable speed drive units are commonly utilized. Both designs targeted removing 95% of the grit contained within the slurry. Traditionally, these units only run about 15–20 minutes every 3 to 4 hours, with timing based upon how often the upstream process equipment flowing into the unit was set or required to run.
More recently, wastewater plants are requiring that smaller grit sizes of 100 mesh or even 150 mesh (approximately 149 to 96 microns, respectively) be removed from the influent waste stream, especially with Combined Sewer and Overflow (CSO) plants and combined systems becoming more prevalent. Plants have also dramatically cut back on the maintenance staffs, so the need to reduce maintenance efforts is increased. CSO systems deliver significantly higher flow at more common intervals and for longer periods of time. The very small grit particles such as 100 or 150 mesh are much more difficult to remove as they tend to stay suspended and do not settle out unless very large tank liquid volumes are utilized to allow enough detention time within the tank for separation to occur. Also over the last 10–15 years, Vortex grit separation has now moved into becoming the more commonly utilized first stage method to handle larger flows and smaller grit particles and is run generally on a continuous basis during storm events. What this has also done is to begin replacing the previously popular and commonly used grit cyclones as the first stage separator of the grit slurry. The Vortex systems typically preferred in CSO applications, do not have many of the problems associated with cyclones, such as routine plugging and rapid wear of the underflow (apex) end of the cyclone.
Without the use of cyclones, the inlet feed rates to the washers and classifiers can commonly become 200–400 GPM. In order to keep overall equipment (tank and screw) size within reasonable limits and to be able to handle the 100–150 mesh particles, a new design for both the classifiers and washers is desired.